Technical Field
The present invention relates to a power supply apparatus installed such as in a vehicle.
Related Art
For example, a well-known in-vehicle power supply system has a configuration that includes a plurality of batteries (e.g., a lead battery and a lithium-ion battery). These batteries are differently used to supply electric power to various electrical loads mounted to the vehicle. Specifically, in the configuration, a rotary electric machine having a function of generating electric power is connected to a lithium-ion battery and a lead battery via a connection path which is provided with a semiconductor switch. The semiconductor switch is opened/closed to electrically disconnect/connect the rotary electric machine from/to the lithium-ion battery and the lead battery. Thus, the lead battery having low durability against frequent charge/discharge (accumulated amount of charge/discharge) is prevented from early deterioration.
Japanese Patent No. 5471083 discloses a technique, according to which, a point where the open-circuit voltage of a lead battery coincides with that of a lithium-ion battery (second battery) is provided in the ranges of use of residual capacity of the lead battery and the lithium-ion battery. According to the technique, battery characteristics are determined so that a relationship of “Li open-circuit voltage>Pb open-circuit voltage” is satisfied on an upper limit side of the point in the range of use of residual capacity of the lithium-ion battery. Thus, while eliminating use of a DC-DC converter to reduce cost, which has been essential in the conventional art, the amount of current flowing from the lithium-ion battery to the lead battery is ensured to be minimized to avoid overcharge of the lead battery.
However, according to the technique disclosed in Japanese Patent No. 5471083, the range where “Li open-circuit voltage>Pb open-circuit voltage” is satisfied and the range where “Li open-circuit voltage<Pb open-circuit voltage” is satisfied are both present within the normal range of use of the second battery. Therefore, in a state of “Li open-circuit voltage<Pb open-circuit voltage”, discharge can no longer be preferentially performed from the lithium-ion battery side. This causes a concern that the lead battery would be deteriorated due to the increase of the accumulated amount of discharge of the lead battery.
There is a measure that can be taken against the decrease of the accumulated amount of discharge in the lead battery when the range satisfying “Li open-circuit voltage>Pb open-circuit voltage” and the range satisfying “Li open-circuit voltage<Pb open-circuit voltage” are both present within the range of use of the second battery. As such a measure, for example, the lead battery and the lithium-ion battery may be disconnected from each other (the semiconductor switch in the connection path may be turned off) when the state turns to “Li open-circuit voltage<Pb open-circuit voltage”. In this case, however, the semiconductor switch is repeatedly turned on/off, which leads to a probability of causing turn-off failure in the semiconductor switch or imperfect control over the semiconductor switch. There is a concern that such turn-off failure induces power supply loss for the electrical loads. To cope with such power supply loss, a backup circuit is required to be provided to ensure power supply, which however causes a concern of increasing cost.